1. Field of the Invention
The present invention relates to a phase-change Random Access Memory (RAM) containing a substrate, a lower electrode, a phase-change material, an upper electrode and a thermal dissipation layer, and particularly, to a phase-change RAM enabling high speed operation with a low current and having improved reliability.
2. Background of the Related Art
Development of next generation memory technologies to replace memory devices such as DRAMs, SRAMs, FLASH memories or the like in the field of semiconductor memories is actively being pursued. According to this trend, much attention is being devoted to a phase-change type RAM that employs a change of resistance which has simple structure and superior properties as compared with other types of next generation memories. Depending on upcoming developments, the phase-change RAM can be expected to replace FLASH memories and may further replace DRAMs and SRAMs, and accordingly to ultimately replace even auxiliary memory units.
The phase-change RAM, which applies heat to a particular material to change its phase and uses the change of its resistance, is a next generation type memory semiconductor for storing data by determining the resistance change according to the phase-change of the material. This phase-change RAM can facilitate the ensuring of operational properties as a stable memory. Also, the phase-change RAM has a characteristic that is not sensitive to changes caused by external conditions. For application as an optical memory, data writing and deleting are performed by heating the particular material using a laser beam to thusly change its phase. For application as an electrical memory, data writing and deleting are performed by heat generated by supplying current. Research for the phase-change RAM using the phase-change material has progressed for several years, but a commercialized product has not yet been released, and merely prototype memories for testing purposes are being fabricated.
The phase-change RAM under development uses a silicon dioxide film (SiO2) for dissipating heat that is generated from the phase-change material to the exterior of a device, as well as for providing electrical insulation between an upper electrode and a lower electrode. However, to use the SiO2 layer as an insulating layer, it has been found that the heat generated in an interface between the phase-change material and the lower electrode is not adequately dissipated to the exterior via the SiO2 layer positioned at an upper end of the lower electrode, but rather such heat is directly transferred to the interior of the device through an electrode material having a superior heat conductivity. Thus, when heat is not properly dissipated to the exterior but transferred to the interior of the device, undesirable heat accumulates within the device when set and reset operations are repeatedly performed, which may cause breakdown of the device. In addition, the SiO2 layer does not quickly dissipate heat to thereby increase the temperature of the overall device, and accordingly it takes a long time to cool the phase-change material, leading to undesirable lengthening of the reset time. Furthermore, when the reset operation is executed at a portion of the phase-change material, the reset is changed again in a set manner, which may cause problems during high speed operation and degrades the reliability of the device.
On the other hand, to achieve low power consumption and high integration of the phase-change RAM, it is preferable to reduce the surface area of the phase-change material which contacts with an electrode to thus generate the desired phase-change. However, due to a limitation of photolithography, there is a limitation in reducing the contact area in order to reduce power consumption. Accordingly, the characteristics of the electrode material and the thermal dissipation layer need to be improved to thusly obtain optimal properties of the phase-change material as much as possible, so as to actually improve the overall characteristics of the device.